The present invention relates to an apparatus and method for recovery of waste heat in a boiler system, and more particularly, to an apparatus and method of waste heat recovery that can be use in a hot water tank.
A boiler is a closed vessel in which water or other fluid is heated under pressure to create steam. The ability of steam to carry large amounts of thermal energy is the property that makes it an effective working fluid. The generated steam is then circulated out of the boiler for use in various process or heating applications. In a conventional boiler system, for example, water is heated under pressure in a boiler, creating steam that can be use in a process equipment such as a turbine. However, in this open system much of the heat energy can be lost during the process.
In a boiler system as described, when the properties (i.e. temperature, pressure, volume) of the working fluid is changed as a consequence of work or heat exchange, then it is said that the fluid has gone through a “process.” In some processes the relationship between pressure, temperature, and volume are specified as the fluid goes from one thermodynamic state to another. The most common processes are those in which the temperature, pressure, or volume is held constant during the process. If the fluid passes through various processes and then eventually returns to the same state it began with, the system is said to have undergone a cyclic process.
In a general cyclic boiler system, the thermal efficiency of the boiler can be increased by the recovery of waste heat. In such a closed loop system, after water enters a boiler where it is heated by an external heat source to become steam, the pressurized steam can be applied to a steam-utilizing device to generate power output, for example. This results in a decrease in the temperature and pressure of the steam vapor. To recover the waste heat, the vapor is collected in a condenser where it is cooled to become saturated liquid, i.e. warm feed water. This warm feed water is then pumped back into the boiler and the cycle is repeated. Because the waste heat from the steam vapor is recycled as warm feed water back into the boiler, the thermal efficiency of the system is improved.
One method of recovering waste heat is disclosed in U.S. Pat. No. 6,196,163 to Shah. In the Shah patent, heat is recovered by capturing the hot condensate via a reactor steam jacket, and then recycling the hot condensate back into the boiler. More specifically, high temperature steam condensate is collected from a reactor steam jacket into a closed vessel in which the pressure is that of the lowest pressure in the reactor steam jacket, but higher than atmospheric, such that there is little or no steam flash over due to pressure drop. The steam condensate is then fed back into the boiler. According to Shah, the amount of heat saving to the boiler is the amount of flash over steam saved and hotter condensate returned to the boiler by this method.
However, the invention described by Shah suffers from a number of disadvantages. For instance, an elaborate system of pumps is necessary for operation in the Shah method, which is not only more costly, but also consumes extra energy to operate and thus results in decreased overall efficiency of the system. More significantly, the boiler cycle described by Shah experiences significant heat loss while the condensate waits in the feed water tank to be fed back to the boiler.
The present invention utilizes the heat of the hot condensate water, which otherwise would be lost through dissipation to the air as low-pressure steam, for other applications. In contrast to the Shah patent, which is directed to a method for recapture of lost heat by preventing hot condensate water from flashing through an elaborate set-up of sophisticated equipments, the present invention prevents hot condensate from flashing by shifting the excessive heat for use to another system, such as a hot water heater, without the need for complicated equipments.
More generally, in systems wherein waste heat is typically recycled back into the boiler to improve the boiler efficiency, heat is still wasted at the stage when the process steam condenses to water and waits in the feed water tank to be pumped back into the boiler. While waiting in the feed water tank, heat dissipates into the environment. However, the heat energy that is lost from the hot condensate in the boiler system can be used for applications in other systems if it is captured before dissipation occurs. In the present invention, the waste heat energy recovered in the form of hot condensate feed water can be used to heat makeup water for a hot water tank.
For the foregoing reasons, there is a need for a method and apparatus for recovering waste heat in a boiler system that can be redirected for use in other applications, such as for use in a hot water tank.